Harbour sites for LNG or large oil tankers are considered to be very hazardous. Therefore, it is not advantageous to place the sites in the vicinity of populated areas. At the same time, the largest number of consumers of LNG is found in densely populated countries. A number of solutions have therefore been suggested to place LNG storage installations at sea.
Further, to transfer LNG, articulated arms or hoses that are well insulated and flexible are often used. The hoses are often in fact very rigid and very inflexible. The articulated arms move normally in one plane only and do not tolerate sideways movements. This requires that a LNG vessel must properly be moored in protected harbours both during loading or unloading operations, lying leeward of the prevailing direction of wind and/or waves.
It has previously been proposed to provide harbour sites for LNG loading at sea that either float or are placed, resting on the ocean bottom. The floating sites have the problem in common that the transfer of LNG between vessel and storage installation takes place between two floating, movable bodies, moving more or less independent of each other. The dynamics put great demands on equipment and safety if the loading takes place side by side.
A major problem of storage structures for liquids resting directly on the sea bed by gravity (GBS=Gravity Based Structure), especially in shallow waters, is that a GBS requires large volumes of fixed ballast to secure positive ground pressure at all times,—also in extreme conditions with e.g. storm surges. It is well known that storm surges mostly appear in shallow waters near land, e.g. in connection with tropical cyclones, where water levels near shore may temporarily increase by up to 8-9 meters. This will expose huge uplift forces onto a GBS with liquids storage with large water plane area at sea level and being located near shore. The additional fixed ballast volumes to counteract such temporary uplift forces will necessitate significant increase of the GBS volume and weight to secure positive bottom pressure at all times, but also to secure additional buoyancy during float-in, submergence and installation of the GBS onto the seabed. Such increase in volume will again result further increase of uplift forces, necessitating additional ballast volumes for both sea water ballast and fixed ballast,—representing a negative design effect spiral which will be make a GBS solution very costly.
It is also known that GBS solutions may not be feasible or in best cases will be very expensive for use in soft and unconsolidated seabed soils, such as found in river deltas. For such reasons the GBS may be equipped with suction skirts, but the mere size and vertical height of such skirt solutions may represent prohibitively expensive foundation solutions, having to date made floating storage bodies the only viable solution in areas with such soil conditions.
An alternative is to transfer LNG between the aft and bow of the two floating bodies, but this is considerably more difficult than corresponding, prior art loading operations for oil, and the method places great demands on the equipment. If in addition these vessels are allowed to rotate, the storage vessel for LNG must be equipped with a complex underwater swivel system for LNG.
To reduce the problems associated with the dynamics of the floating bodies during loading operations, it has been proposed to install large, rectangular steel or concrete structures on the seabed, functioning as artificial harbours, where a continuous steel or concrete wall is intended to form a protection for incoming waves. Typical depths of water proposed are 8-30 metres. This type of large constructions are intended to be built away from populated areas and at the same time functioning as a breakwater for the LNG ships during loading and unloading operations.
The problem can be reduced by moving the ship over onto the leeward side of the harbour construction, but calculations and basin experiments have shown that the harbour construction which forms a continuous barrier must be built to be very large if one is to obtain a significant shielding effect when waves and swells come during one period from a particularly unfavourable angle. This is due to the well known effect that ocean waves will be bent around both sides of such a construction and a focal point will arise some distance behind the leeward side where the bent waves meet. At this focal point, the height of the waves can actually be higher than the incoming waves.
A large harbour construction placed on the ocean bottom, intended to act as a shield from the waves, will therefore be very costly. Different forms for such types of harbour sites for LNG built in concrete for shielding vessels from the waves during loading operations have been suggested. One suggested shape is, for example, to build the construction as a horseshoe and let the LNG vessels load/unload inside this. This will reduce the dynamics considerably, but the harbour site will be even more costly than a harbour site in the shape of a rectangle.
GB 1369915 describes a harbour site comprising a number of units that are afloat or sunk and otherwise constructed for placement on the seabed. Each unit comprises a base, load-carrying structure and moveable wave-breaking elements that can be moved if required.
U.S. Pat. No. 3,958,426 describe a harbour site comprising a number of units placed apart on the seabed, so that at least one straight mooring location is formed. The units are provided with fenders and wave dampening devices.
Applicants own publication WO 2006/041312 discloses a harbour plant for storage, loading and unloading hydrocarbons such as LNG at sea, the whole content of which hereby being included by the reference. The harbour comprises three units built from steel or concrete, placed on the seabed. The units are placed in sidewise relation in-line. The harbour is configured to dampen the waves, the vessel being intended to lie on the leeward side of the mooring.
Applicants own publication WO 2013/002648 discloses a harbour plant for storage, loading and unloading of hydrocarbon products at sea, comprising a number of units being mutually placed on the seabed so that a harbour plant is formed. The units are placed independently at a given distance apart in sideways direction and having a front surface along which a vessel is intended to be moored, forming passage(s) for parts of the waves, and being configured to dampen a part of the incoming waves while allowing other parts of the waves and current to pass through the harbour plant.
US 2005/139595 describes a plant storage and loading LNG, consisting of a seabed structure resting on a seabed, the seabed structure having a base slab resting on the seabed and three upwards extending walls. The seabed structure has an opening, allowing a floating module to be manoeuvred into position inside the seabed structure and ballasted to rest on the base slab.
FR 2894646 describes a gravity based structure resting on the seabed due to its own weight and provided with downwards projecting and open skirts, pressed down into the seabed. The gravity based structure has a U-shaped form, with vertical walls extending upwards from a submerged bottom slab, provided with buoyancy chamber, functioning as weight for providing the required weight. One embodiment of the gravity based structure may also be provided with piles extending downwards through the vertical walls and into the supporting soil, the piles being terminated at the top of the walls above sea level.
However, these harbour plants for storage can be large in scale, complex and expensive. They take a long time to build and they have limited variation with respect to mobility and other applications. Due to dependencies of deep skirts to enable foundation, problems may also be experienced during installation, in particular in shallow waters with muddy or soft seabed. In addition, the density, composition, consolidation and topography of seabed soil may vary significantly for one seabed location to another. For example, the soil in river mouths will often be dominated by soft, muddy soil with a kind of yoghurt texture, while other seabed areas may be influenced or overlapped by hard sandstone, limestone or ancient volcanic rock. This will have direct impact on the load bearing capacity of the seabed soil, and hence the possibility to find a predictable and reliable foundation solution for a seabed structure which shall be resting onto the seabed.
Hence, there exists a requirement for cost-effective, versatile and flexible harbour plant systems that can store different oil related products and bunkering, and are easy to build, maintain and repair, and which can be standardized as far as possible for fabrications and cost reasons, and which can easily be deployed in offshore or near shore locations with any type of seabed soil.